The nature of the exalted Kekulé vibration of styrene and other benzene derivatives in the S1 state

Two-photon spectroscopy is used to measure the frequency of the Kekulé vibrational frequency of several benzene derivatives in the first excited state (S1). It is found that the frequency is always increased with respect to the ground state, and that it is essentially constant. This finding is in line with a model presented by the authors, in which the ground and first excited states of benzene are treated as `twin statesʹ. The physical nature of this vibration is discussed in view of quantum chemical calculations.